Bleed-off control is widely used in hydraulic controllers for working machines to control the speed of an actuator by bleeding off a flow of hydraulic fluid from a hydraulic pump to a tank through a center bypass line of a directional control valve. The directional control valve, in the bleed-off control, starts to close a bleed-off opening, which is connected to the tank from a meter-in opening point. The meter-in opening, connected to the actuator, starts to open and reduces the bleed-off opening while increasing the meter-in opening according to a spool stroke to a bleed-off closing point, where an entire flow from the hydraulic pump is supplied to the actuator. When pilot hydraulic pressure, which is a linearly-increasing function in relation to a manipulated variable of a working machine lever, is supplied to a pilot portion of the directional control valve from a pilot proportional control valve, a spool of the directional control valve makes a stroke according to the pilot hydraulic pressure. Therefore, a rate of flow of pressurized oil, supplied to the actuator, changes according to the pilot hydraulic pressure, and the speed of the actuator is controlled.
Characteristics of the directional control valve in the bleed-off control will be described with reference to FIG. 14. A manipulated variable Ls of the working machine lever is shown along the horizontal axis. From a manipulated variable O at a neutral point (hereinafter referred to as a neutral point O), wherein a meter-in opening Ami is fully closed and an entire flow from the hydraulic pump is bled off to a manipulated variable Obo at the bleed-off closing point (hereinafter referred to as a bleed-off closing point Obo), where a bleed-off opening Abo is fully closed, as is shown with a continuous line, the directional control valve decreases the bleed-off opening Abo while increasing the meter-in opening Ami, according to a spool stroke of the directional control valve. With the manipulated variable Ls of the working machine lever shown on the horizontal axis and the pilot hydraulic pressure pp shown on a vertical axis, pilot hydraulic pressure F0, generated by the pilot proportional control valve, satisfies pilot hydraulic pressure pmi in a manipulated variable omi at the meter-in opening point (hereinafter referred to as a meter-in opening point Omi) and pilot hydraulic pressure pbo at the bleed-off closing point Obo, and is shown as a linearly-increasing function in relation to the manipulated variable Ls.
As is described above, the pilot hydraulic pressure F0 is shown as a linearly-increasing function in relation to the manipulated variable Ls, as is shown by a continuous line, and thus the spool stroke of the directional control valve also becomes a linearly-increasing function in relation to the manipulated variable Ls. Incidentally, on the horizontal axis of FIG. 14, the manipulated variable Ls and the spool stroke are shown at the same scale. Therefore, the neutral point O, the meter-in opening point Omi, the bleed-off closing point Obo, and the like are common in both the manipulated variable Ls and the spool stroke. Actuator flow rates Q in a loaded condition and in an unloaded condition, at engine rated speed, and actuator flow rates Q in a loaded condition and in an unloaded condition, at minimum idling engine speed, are respectively shown with continuous lines. When a cargo handling machine such as a bucket rises, actuator driving pressure P changes to pass actuator driving pressure P1, at an actuator starting point m1 in unloaded condition, and actuator driving pressure P2, at an actuator starting point m2, in loaded condition, as is shown with dashed lines.
(1) When a flow rate of pressurized oil, passing through the bleed-off opening Abo, is represented by Q, a difference in pressure before and after the bleed-off opening Abo is represented by .DELTA.P, and a flow coefficient of a bleed-off opening is represented by C, it is known that the expression (1) applies: EQU Q=CAbo.multidot.p.sup.1/2
As the engine speed is decreased at a minimum idling engine speed, the discharge quantity of hydraulic fluid from the hydraulic pump, that is, the flow rate Q of pressurized oil flowing into the bleed-off opening Abo, decreases. It is necessary to reduce the bleed-off opening Abo to hold a predetermined actuator driving pressure P (P1 in an unloaded condition, and P2 in a loaded condition) even when the flow rate Q decreases, as can be seen in expression (1). Specifically, at an actuator starting point, the manipulated variable Ls of the working machine lever increases from m1 at an engine rated speed to n1 at a minimum idling engine speed in an unloaded condition, and from m2 at an engine rated speed to n2 at a minimum idling engine speed in a loaded condition.
(2) When engine speed is fixed and the flow rate Q of pressurized oil passing through the bleed-off opening Abo is fixed, as the working machine changes from an unloaded condition to a loaded condition, the actuator driving pressure P, for starting the actuator, increases from P1 in an unloaded condition to P2 in a loaded condition. Therefore, as can be seen in expression (1), it is necessary to make the stroke of the spool so that the bleed-off opening Abo will be reduced. Specifically, the manipulated variable at the actuator starting point increases from m1 in an unloaded condition to m2 in a loaded condition at an engine rated speed, and from n1 in an unloaded condition to n2 in a loaded condition at minimum idling engine speed.
The aforementioned prior art, however, has the following disadvantages.
(1) When an engine changes from an engine rated speed to a minimum idling engine speed, the discharge quantity of the hydraulic pump decreases and the flow rate Q of pressurized oil flowing into the directional control valve decreases. The manipulated variable Ls of the working machine lever increases from m1 to n1 at the actuator starting point in an unloaded condition and from m2 to n2 at the actuator starting point in a loaded condition. Moreover, as the working machine changes from an unloaded condition to a loaded condition and the actuator driving pressure P increases from P1 in an unloaded condition to P2 in a loaded condition, the manipulated variable of the working machine lever at the actuator starting point increases from m1 to m2 at an engine rated speed, and from n1 to n2 at minimum idling engine speed. Hence, there is a disadvantage in that a dead zone of the working machine lever to the actuator starting point increases.
(2) In a simultaneous manipulation in which an actuator load on a downstream side is larger than an actuator load on an upstream side, and when the difference between both of the above actuator load increases, most of the pressurized oil from the hydraulic pump flows to the actuator on the upstream side, whereby the quantity of pressurized oil of the actuator on the downstream side is decreased. Thus, the quantity of oil pressure on the downstream side is obtained by narrowing the bleed-off opening of the directional control valve on the downstream side to be in an almost fully closed condition while narrowing the bleed-off opening of the directional control valve on the upstream side, and by also increasing the meter-in opening. Accordingly, there is a disadvantage in that operability is lowered, since the manipulated variable of the directional control valve on the downstream side increases.
(3) When the flow rate Q of pressurized oil flowing into the directional control valve changes with a chance in the discharge quantity of the hydraulic pump, depending on engine speed, or the actuator driving pressure P chances depending on a working state of the working machine, the actuator starting point changes a substantial amount from ml and m2 at an engine rated speed to n1 and n2 at a minimum idling engine speed. Therefore, an operator needs to frequently revise the manipulation of the working machine lever depending on engine speed or load pressure of the actuator while watching the movement of the working machine, whereby there is a disadvantage in that operability is lowered.
(4) If the bleed-off opening Abo is reduced in size without changing the size of the meter-in opening Ami of the directional control valve in relation to the manipulated variable Ls, in order to decrease the manipulated variable Ls so that the actuator starting point will be m1 and m2 at an engine rated speed and n1 and n2 at a minimum idling engine speed, there is a disadvantage in that the flow rate Q, supplied from the meter-in opening Ami to the actuator, increases a substantial amount when the discharge quantity of the pump increases. If the meter-in opening Ami and the bleed-off opening Abo arc reduced in order to prevent the aforesaid disadvantage, there is a disadvantage in that the pressure loss in both openings Ami and Abo increases.